The invention provides a mining method of undersea natural gas hydrates and a device thereof. The undersea natural gas hydrates are mined by utilizing renewable energy sources, abundant solar energy in marine areas is collected and transformed into electric energy which is transformed into heat energy for heating a stratum containing the natural gas hydrates which is a certain depth from a submarine stratum so as to lead the hydrates therein to be decomposed, and the natural gas generated from decomposing is collected. The device comprises a solar power generating system arranged on a sea production platform and a distributor comprising an electric heater, wherein, the solar power generating system is connected with the electric heater by an insulated cable so as to supply power for the electric heater. The method has the advantages of high efficiency, low energy consumption, strong feasibility, and safety and environmental protection; in addition, the device is simple and the cost islower.